The present invention relates to a grease composition for use in constant velocity joints, in particular, for ball type fixed and plunging constant velocity joints. A very high contact pressure is developed between the parts of the constant velocity joints to be lubricated and the joint parts undergo complicated rolling and sliding motions. This often results in abnormal wear and metal fatigue and, in turn, leads to a spalling phenomenon, i.e., pitting of the joint parts. More specifically, the present invention relates to a grease composition for constant velocity joints which can effectively lubricate such constant velocity joints to effectively reduce the wear of joints and to effectively reduce the occurrence of any pitting of the parts.
Examples of lubricating greases conventionally used in such constant velocity joints include a lithium soap thickened extreme pressure grease containing molybdenum disulfide and a lithium soap thickened extreme pressure grease containing molybdenum disulfide and extreme pressure agents, e.g., sulfur-phosphorus or a lead naphthenate. However, these greases for constant velocity joints have not always been satisfactory in the severe working conditions which occur in the present high-performance motorcars.
Recently, the number of four-wheel drive (FF-type) motorcars have rapidly increased. For use in these vehicles constant velocity joints need to be as making light and small as possible. The double offset type constant velocity joints and cross groove type constant velocity joints used as the plunging joints as well as Birfield joints used as the fixed joints have a structure in which torques are transmitted through 6 balls. These joints cause complicated reciprocating motions such as complicated rolling and sliding motions during rotation under a high contact pressure, stresses are repeatedly applied to the balls and the metal surfaces which come in contact with the balls and accordingly, the pitting phenomenon is apt to occur at such portions due to metal fatigue. The recent improvement in the power of engines is accompanied by an increase in the contact pressure as compared with conventional engines. Motorcars are being made lighter to improve fuel consumption and the size of joints has correspondingly been down-sized. This leads to a relative increase in the contact pressure and thus the conventional greases are ineffective in that they cannot sufficiently reduce the pitting phenomenon. In addition, the greases must also be improved in their heat resistance.